Ram-type and annular BOPs are part of a pressure control system used in oil and gas drilling operations to control unexpected well bore pressure spikes or “kicks” as they are commonly referred to in the industry. Well bore kicks can cause the well bore pressure to reach several thousand pounds per square inch during a kick and can cause severe equipment damage as well as physical harm.
The ram-type BOP has a body with a vertical bore and a pair of laterally disposed opposing bonnet assemblies. Each bonnet assembly includes a piston which is laterally moveable within the bonnet assembly by pressurized hydraulic fluid. Replaceable sealing elements called “packers” are mounted within rams attached to the ends of the pistons which extend into the BOP bore. When these pistons are moved to a closed position, commonly referred to as “closing the BOP” or “closing the rams,” the vertical bore of the BOP is sealed and the kick is contained. These packers are available in a variety of configurations designed to seal the BOP bore when the opposing rams and pistons are moved to their closed position. For a ram-type packer, each ram packer typically has a semicircular opening in its front face to form a seal around 180° of the outer periphery of the pipe. When the rams are closed, the opposing ram packers meet and seal the entire 360° periphery of the pipe.
The annular BOP utilizes a hemispherical piece of rubber usually reinforced with inserts. Unlike a ram-type BOP which closes with a horizontal motion, an annular BOP closes inward around the drill string in a smooth simultaneous upward and inward motion to seal on the pipe or the open hole. The geometry of this movement reduces internal stresses and friction between the BOP body and the sealing element, which translates into a longer field life with less maintenance. The annular design may also operate with a much lower operating pressure, reducing the number of hydraulic accumulators necessary, and thereby reducing cost and complexity of the BOP.
Packers that can be used for either a ram-type or annular BOP can be designed to seal around pipe of a specific size in the blowout preventer bore when the blowout preventer is closed. Other packers though may be configured to seal around a range of pipe sizes, and are referred to as variable bore packers. Both packers form a pressure tight seal during a kick until the well bore pressure can be controlled.
To form a proper seal with a variable bore BOP, the packer material must be of a low enough Durometer to close against the pipe and provide enough pressure for a range of pipe diameters. However, a low Durometer also tends to make the packer suffer from lack of support during the loading process. Increased Durometer packers may be used but more force is needed to form a seal, sometimes resulting in an inadequate seal due to incomplete closure and/or low pressure. Additionally, the increased Durometer packers may not be able to seal against as wide a range of pipe diameters. With both the ram-type and annular variable bore BOPs, the variable bore packer thus typically includes an annular or two semi-circular elastomeric sealing elements with an array of support inserts embedded in the elastomeric material. The inserts are molded within the elastomeric material in a pattern around the opening of the elastomeric material, forming unitary structure. The structure allows the plurality of packer inserts to move and seat against different diameter tubular members and also helps prevent extrusion of the elastomeric material between the packer inserts and the tubular member.
Even with inserts, however, some variable bore packers still have durability issues. Although inserts help prevent extrusion, the inserts are still configured and designed to adjust for sealing against different diameter pipes. Thus, the inserts still include gaps between the insert tips and the pipe being sealed against when the seal is actuated. When subject to load, the packer elastomeric material may still extrude through these small gaps, causing the packer material to tear and break apart and thus lose ability to form an adequate seal.